Effect of tropical environments on poultry production and related requirements for appropriate housing is a much talked about and written on subject, with the tropical environment often considered a single, uniform entity.

In reality, it exhibits a wide range of conditions related to latitude, altitude, and climatic factors including temperature, humidity, solar radiation and air movement. Poultry producers in tropical countries must design their houses according to the exact conditions experienced within their tropical zone.

Environmental stress

The physiology of domestic poultry and how it is affected by physical factors in relation to environmental stress is complex.

First and foremost, the internal body temperature of domesticated gallinaceous birds (chickens) at 41.2-42.2 C is measurably higher than that of mammalian livestock and humans (36-39 C). Poultry have considerably less threshold to heat stress as compared with other animals. In comparison, the body temperature of poultry can fall as much as 20 C below the normal range with birds making a full recovery if carefully re-warmed. Domestic poultry are much more likely to die from heat stress (hyperthermia) than of stress associated with low temperature (hypothermia).

Poultry are not well adapted to high, ambient air temperatures because they lack sweat glands in the skin to gain much from natural evaporative cooling, although there is some direct movement of water by diffusion through the skin tissue. Only the head appendages (e.g. comb) are rich in blood vessels and able to allow direct loss of heat.

Blood vessels in the facial appendages and the legs vasodilate (come closer to the surface of the skin) to enable sensible heat loss (conduction, convection and radiation) from the body in addition to insensible heat loss through evaporation. Sensible heat loss contributes to a warming of the poultry environment whereas evaporation takes heat out of the environment by absorbing the latent heat of evaporation.

Most of the heat loss achieved by hens occurs through the respiratory system and, more specifically, the lungs, which have large, non-vascular air sacs which play no role in gaseous exchange of oxygen and carbon dioxide. Instead, they present large areas of wet surface for loss of water by evaporation and therefore, evaporative cooling. Birds lose this heat through panting, which happens when their body temperature reaches 43 C.

The negative effects of high humidity pose additional problems. Air saturated with water vapour prevents further loss from birds through evaporation. The role of relative humidity is closely associated with temperature. At around 20 C, relative humidity can be raised to 90% with seemingly little effect on either food-conversion efficiency or growth rate, but at 29 C, raising the humidity to around 70% has a discernible inhibitory effect on the rate of growth of broiler birds.

Poultry metabolism

Birds continually generate heat through body metabolism but during high, ambient air temperatures of 35-40 C, they are unable to discharge heat through evaporation. Body temperature starts to rise, causing more heat to be produced and if unchecked, results in acute heat stress and death from hyperthermia.

The act of feeding, including moving around to find food, ingestion, digestion and assimilation, generates heat. So, a bird's metabolism reacts by reducing food intake. Water intake may double initially but slips back to normal levels. There are documented negative effects of heat stress on layers including egg production, egg weight and shell weight, and on broilers with increases in total carcass and abdominal fat.

Poultry housing to suit

The overall effect of the tropical environment is usually greatest on high-producing hybrid hens, which have been bred for production in temperate regions where the consequences of low-temperature-induced cold stress are more important.

Hybrid birds are invariably raised in appropriately designed houses and there is often temptation to copy and adopt such designs for the tropics.

But these houses may be inappropriate for prevailing environmental conditions as well as the economic conditions of production in a tropical environment.

These designs are unable to keep out sufficient heat through insulation or create the necessary degree of air flow to achieve the right in-house climate for maximum productivity.

Two distinct climates

Two climatic areas of the tropics need to be looked at.

First are those areas lying up to 15 degrees on either side of the Equator, experiencing temperature ranges of 25-35 C with exceptionally high humidity and accompanying small air movements.

Second are the classic hot, arid or semi-arid areas experiencing a wider range of temperature, from 8-49 C, with distinct and marked diurnal (day/night) and seasonal (summer/winter) variations. Such regions which typically occupy the outer tropical zone (15-30 degrees north and south of the Equator) receive high intensities of solar radiation but considerably more frequent and higher winds.

House location

The first consideration is where to locate the poultry house. As a general rule, house sites should take full advantage of local topography so that maximum use is made of prevailing air currents and shading from high land masses. Consult local records of wind velocity and direction and orientate houses to benefit from prevailing air movements. Always locate the house well away from concentrated blocks of tall plantation tree crops such as rubber and other buildings that could interfere with natural air flow into and through the poultry house.

Tall trees with bare trunks, foliage in crowns and located sparingly near the house can provide good shading benefits without interfering with air flow. Small, bushy trees and shrubs such as citrus and mango have an overall negative effect because they are not tall enough to provide shade and block the air flow.

Appropriately located trees and other vegetation may complement cooling of the house through transpiration of water from the leaves, taking heat from the immediate environment for evaporation. Air movements across transpiring crops in hot climates can reduce ambient air temperature by up to 3 C.

In hot, dry areas, poultry house location near crops, including pasture under irrigation, can bring considerable rewards. A poultry house that is completely surrounded by grassland or an alternative low profile cover crop like alfalfa, groundnuts, cowpeas (Vigna) or cucurbits (such as cucumber, pumpkin or musk melon) absorb solar radiation, whereas bare ground and soil will reflect considerable amounts onto the poultry house.

It is not a good idea to locate poultry houses near dense natural vegetation or crops such as sugar cane and cocoa which may harbour rodent predators like brown rats.

If raising ducks, always reserve an area outside the house, an equivalent to half the house area, to construct a pond of shallow water which need be no more than 30cm deep. With this supplied, ducks can often withstand high ambient temperatures that would otherwise cause high mortality in chickens and turkeys.

Clearly there are no hard and fast rules. It's more a question of "mix and match" to suit.

House construction

Roof design and construction in the tropics must ensure total interception of solar radiation to prevent direct heat transference onto birds in the poultry house. Shade is crucial and can halve the heat load on hens.

Insulation of galvanised iron roofs is essential because it reduces transference of long-wave radiation down onto the birds. Grass or other dried thatch may seem a cheap choice but care must be exercised because it may harbour vermin, including rodents and wild birds, as well as avian parasites such as smites and ticks.

The colour and nature of a roof surface will have considerable impact on the amount of solar radiation absorbed with white and smooth, shiny surfaces staying cooler than dark coloured and rough surfaces. Painting the roof white is a good idea not only because of the colour but also as an anti-corrosion measure to stop development of a rough surface coating of rust.

Regular movement of air within the house takes away heat, water vapour and pollutant chemicals, including ammonia from the faeces. If the birds are in battery cages, continual air flow is crucial to stop those in cages that are situated higher from getting too hot.

In some climates, it may be necessary to have one permanent solid wall to shield birds from strong wind and driving rain that can occur any time of the year. In others, a temporary facility that can be erected or taken down easily may be all that is needed to protect birds during three months of annual rainy season.

Houses for relatively small numbers of birds in the hot, dry tropics with broad daily fluctuations in temperature benefit from walls with high heat capacity and low heat conductivity. They absorb heat during the day and release it at night, equalising house temperature over the 24-hour cycle.

This is not advisable for poultry units housing large numbers of birds as there will be insufficient air flow to prevent accumulation of ammonia.

Larger units

Large units were traditionally built as open-sided structures to encourage maximum natural air movement, which is both cheap and reliable.

Now, a larger numbers of enclosed houses are being used, which clearly require environmental controls whether from ventilation systems providing continuous air flow, high pressure misting systems or fogging/cool pad/tunnel ventilation systems which rely on a combination of evaporative cooling and air flow.

Installation of such systems requires a higher initial investment. Poultry producers moving towards environmental control should also keep an eye on reliability of electricity supply and build the cost of a standby generator into their calculations.